The present invention relates to a data transfer system having a host computer and a terminal unit connected together by, for example, a Centronics cable.
In a conventional data transfer system having a personal computer (hereinafter referred to as a PC) as a host computer and a terminal unit which are connected together by a Centronics cable, when the data is transferred from the PC to the terminal unit, the data transfer rate is adjusted by the PC. In a parallel interface between the PC and the terminal unit of such data transfer system, a parallel port signal as defined in the IEEE 1284 standard is employed for the data transfer. The IEEE 1284 standard includes a compatibility mode (in forward direction) which is used to transfer the data from the PC to the terminal unit and a nibble mode (in reverse direction) which is used to transfer the data from the terminal unit to the PC.
During the data transfer from the PC to the terminal unit, if a data processing rate of the PC is above a data processing rate of the terminal unit, the data processing in the terminal unit cannot catch up with the data transfer from the PC. On the contrary, if the data transfer rate is lagging with respect to the receiving capacity of the terminal unit, a full utilization of the capacity of the terminal unit is prevented.
In the conventional data transfer system, the PC repeats trials of transmitting a signal to the terminal unit while changing a strobe interval in accordance with a program in the PC. The terminal unit judges whether or not the strobe interval is long enough to be able to receive the data transferred from the PC in a normal manner. If the terminal unit judges the strobe interval is long enough, the terminal unit transmits a notice of the proper data transfer rate to the PC, whereupon a proper strobe interval is established by the PC. On the contrary, if the terminal unit judges the strobe interval is too short, the strobe interval is changed by adding a given increment time, whereupon the PC repeats trials of transmitting a signal to the terminal unit.
However, the PC is not provided with a timer which is able to measure a sufficiently small increment time used for changing the strobe interval during the repetitive trials of transmitting the signal to the terminal unit. Therefore, the conventional data transfer system cannot use a proper increment time for changing the strobe interval.
If too small increment time is used for changing the strobe interval, the number of trials of transmitting the signal to the terminal unit must be increased, resulting in an increased length of time until the PC receives the notice of the proper data transfer rate. On the contrary, if too large increment time is used for changing the strobe interval, the long strobe interval, namely, a slow data transfer rate is established, resulting in a failure to take a full utilization of the capacity of the terminal unit.